The general goal of this proposal is to understand the processing of sounds using two ears. The proposed work continues activities that have been ongoing in our laboratory for a number of years, directed toward an integrated understanding of the physiological, psychophysical, and clinical issues involved in binaural and spatial processing. In this proposal, the effort is twofold: first, to use physiological modeling of the brainstem, including the inferior colliculus, to characterize the representation of basic binaural information; and second, to use psychophysical experimentation and black-box modeling to characterize the use of this basic binaural information in complex tasks such as understanding speech in day-to-day environments, which typically include multiple sources and reflecting surfaces. We focus specifically on the advantages (and occasional disadvantages) of using two ears (relative to a single ear) as well as on the neglected contributions of interaural amplitude differences. The experimental program includes measurements of performance both by listeners with normal hearing and by listeners with impaired hearing. Overall, our studies are designed to provide functionally useful computational models of brainstem nuclei, an understanding of what binaural information is represented by brainstem neural activity, an understanding of how this information is used by more central portions of the brain to achieve observed performance in a variety of complex environments, and an understanding of the sources of difficulties for hearing-impaired listeners in these environments. Finally, we hope to sort out what binaural and/or spatially distributed information is useful for the hearing impaired and how to best provide that information to them.